Method and apparatus for producing extensible paper

ABSTRACT

A method for producing extensible paper, comprising the following stages: 
     feeding a mix of vegetable fibres to a kneader member, 
     mixing the mix with water in the kneader, 
     beating the fibres to obtain a pulp, 
     transferring the beaten pulp into a flow chest, 
     feeding the beaten pulp from the flow chest onto a paper web formation cloth with consequent reduction of the water percentage by gravity and vacuum, 
     pressing the web, with consequent further reduction of its water content, 
     initial drying of the paper web to a substantially constant moisture content of between 15% and 65%, 
     compacting, 
     final drying to a moisture content of between 15% and 4%, preferably 10%-8%, 
     glazing, 
     wherein: 
     the beating stage is carried out by rubbing the fibres in a multistage unit to obtain a pulp having a degree of beating of at least 30° SR, 
     the compacting stage is carried out between at least a pair of rollers of which one is of hard material comprising circunferential surface ribs and driven at greater speed, and the other is of soft material with a smooth surface and driven at lesser speed.

FIELD OF THE INVENTION

This invention relates to a method for producing extensible paper and aplant for implementing the method.

DESCRIPTION OF THE PRIOR ART

U.S. Pat. No. 3,454,463 discloses a method of subjecting newsprint paperto controlled compressive shrinking including the features of passingthe wire side of a web in contact with the moving smooth surface of amechanical compactor and shrinking the newsprint web to a valuesubstantially above the desired value and then stretching the web to thedesired value.

EP-A-0631014 discloses a process for making soft tissue products.

An object of the invention is to provide a method enabling paper to beproduced with a high degree of strength while at the same time with anextensibility both in the longitudinal direction and in the transversedirection of practically the same order of magnitude.

A further object of the invention is to provide a method enabling paperto be produced with a continuous plant.

A further object of the invention is to provide a method enablingproducing paper with smooth surface in order to improve its printabilitycharacteristics.

BRIEF SUMMARY OF THE INVENTION

These and further objects which will be apparent from the ensuingdescription are attained according to the invention by a method forproducing extensible paper, comprising the following stages:

feeding a mix of vegetable fibres to a kneader member,

mixing the mix with water in the kneader,

beating the fibres to obtain a pulp,

transferring the beaten pulp into a flow chest,

feeding the beaten pulp from the flow chest onto a paper web formationcloth with consequent reduction of the water percentage by gravity andvacuum,

pressing said web, with consequent further reduction of its watercontent,

initial drying of the paper web to a substantially constant moisturecontent of between 15% and 65%,

compacting,

final drying to a moisture content of between 15% and 4%, preferably10%-8%,

glazing,

wherein:

said beating stage is carried out by rubbing the fibres in a multistageunit to obtain a pulp having a degree of beating of at least 30° SR,

said compacting stage is carried out between at least a pair of rollersof which one is of hard material comprising circunferential surface ribsand driven at greater speed, and the other is of soft material with asmooth surface and driven at lesser speed.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is described hereinafterwith reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a plant for implementing the method of theinvention;

FIGS. 2, 3, 4 and 5 are schematic views showing different rollers usedin the preforming stage;

FIG. 6 is a schematic view showing the rollers used in the compactionstage.

DESCRIPTION OF PREFERRED EMBODIMENTS

As can be seen from the figures, the plant for producing extensiblepaper comprises essentially a high density kneader 1 consistingsubstantially of a cylindrical tank 2 with an inverted frusto-conicalbase and internally housing a conical impeller 3 comprising on itssurface a helical projection.

The kneader 1 is connected to a storage vat 4 provided with a stirrer 5and connected to a beating station 6 formed from beating units 7 of lavadisc type alternating with storage chests 8. The exit from the lastbeating unit 7 is connected to a further high density beating unit 7' ofperforated ring type connected to a storage vat 9 communicating with aflow chest 10 feeding a station 11 in which the paper web is formed.This station comprises a cloth 12 taut between two return rollers 13 andarranged to subject the pulp of water and fibrous raw materials toprogressive water extraction by means of gravity and vacuum.

The downstream end of the paper web formation station 11 leads to apressing station 14, downstream of which there are provided animpregnation station 23 and a successive preforming station 15.

Downstream of the preforming station 15 there is provided a dryingstation 16 of hot roller type which ensures a constant moisture contentof the paper web of between 15% and 65%, preferably 50%, and asubsequent compaction station 17 comprising pairs of counter-rotatingrollers of different type and surface consistency, driven at differentspeeds and able to compact the web both longitudinally and transversely.In particular the upper roller 37 of each pair is of metal, comprises aplurality of surface circumferential ribs 38 and rotates at greaterspeed than the corresponding lower roller 36 which is in rubber and hasits surface smooth.

The exit of the compaction station 17 is connected to a further dryingstation 18, which is connected to a glazing station 39 connected to asuccessive paper winding station 19.

The drying station 18 ensures a water content of the paper web ofbetween 4% and 15%, preferably 10%.

The purpose of the glazing station 39 is to improve the printability andbondability characteristics of the extensible paper obtained andoperates with a linear load of between 10 and 100 kg/cm, preferablybetween 50 and 52 kg/cm.

Between the drying station 18 and winding station 19 there can beinserted a further impregnation station and a further drying station forsubjecting the paper web to treatment to improve its printabilitycharacteristics, if required.

The plant according to the invention also comprises a series of controlsand automation devices which ensure that the operating cycle iscorrectly implemented and which will be mentioned as required during thecourse of the following description.

The operation of the plant according to the invention will now bedescribed with reference to the passage of the paper web under formationthrough the successive stations.

The bales of fibrous raw material are fed to the high density kneader 1together with a predetermined quantity of water for their mixing. Herethe pulp is kneaded, mixed with water and particular substances added,the purpose of which is to increase the ultimate strength of the fibres,to homogenize the water and fibre pulp and give special characteristicsto the paper obtained.

In particular the fibrous raw material consists of vegetable fibreswhich can be long-fibre cellulose, short-fibre cellulose or other fibresobtained from vegetables other than wood (cotton linters, hemp, flax,esparto, kenaf). The different raw materials can be worked on the sameline or preferably on different lines.

Rotating the impeller 3 results in progressive kneading of the fibrousraw material, which preserves the length of the original fibres andresults in their intimate mixing with the water and the additives fed tothe kneader. In particular, the additives used can include starches,which are able to bind the fibres together and increase their ultimatestrength, or carboxymethyl-cellulose (CMC), the purpose of which is todisperse the fibres and hence prevent their coagulation, or syntheticand/or lactic resins the purpose of which is to bind the fibres togetherto form a sort of elastic bond.

A pulp of fibre, water and additives leaves the kneader 1 with a drycontent of about 15%, this pulp being fed to the successive beatingstation 6 to be subjected to the action of the lava disc beating units7, which work the fibres substantially without cutting them, but byhydrating them and by rubbing and conferring particular characteristicson the pulp. As a result of this treatment, the fibres are modified suchas to facilitate their consolidation and to form a continuous andhomogeneous structure, essential for the characteristics which the finalproduct has to present.

The degree of beating of the pulp can be determined on the basis ofobjective parameters measured in SR (Shopper Reagler) units, andaccording to the present invention the pulp leaving the beatingtreatment must be between 30° SR and 60° SR according to the weight ingrames of the paper to be obtained.

At the exit from the last beating stage the pulp, which as stated isbetween 30° SR and 60° SR, is fed into the perforated ring beating unit7', which operates at a density of about 20% and the purpose of which isto hydrate the fibres, to swell and curl them. The pulp is then fed intothe storage vat 9 and from here into the flow chest 10, from which witha dry content of about 0,5-1% it is poured onto the underlying cloth 12of the paper web formation station 11.

On the initial portion of this cloth the pulp tends to progressivelyeliminate water, firstly by gravity and then by suction, until at theexit end of the cloth it has a dry content of about 18%.

The paper web 20 leaving the station 11 passes to the pressing station14 between pressing rollers 21 and felts 22, losing water to attain adry content of about 35%.

The paper web then passes to the impregnation station 23 where it istreated with a solution of various additives the purpose of which is toimprove the extensibility characteristics of the paper and/or to improvethe production technology. This impregnation is preferably effected by aspray device but can also be effected by other systems, for example bypassing the forming paper web through tanks containing the impregnationsolution. In either case the quantity of impregnating substance iscontrollable, with considerable advantages both in terms of cost of thesubstance used and in terms of exact determination of said substance.

The web impregnated in this manner is subjected to preforming treatmentin the station 15, in which one or more preforming units are provided.These can be all identical or different. In particular, each preformingunit can comprise:

an upper roller 24 with a shaped profile and a smooth roller 25 withtraditional smooth felt 26 (FIG. 2);

a smooth upper roller 27 and a smooth lower roller 28, between which afelt 29 is interposed having an external marking structure to suitablydeform the forming paper web interposed between said felt and the upperroller 27 (FIG. 3);

an upper roller 30 with a shaped profile and a smooth lower roller 31(FIG. 4);

a smooth upper roller 32, a smooth lower roller 33, a traditional felt34 and a felt 35 of marking structure interposed between the felt 34 andthe upper roller 32 (FIG. 5).

The use of several preforming units, which can be identical ordifferent, enables profiles practically of any design to be obtained onthe paper web, and in particular designs not obtainable with a singlepreforming unit.

The paper web preformed in this manner is dried in the station 16 to adry content of about 50-60% by passage through hot roller or a hot airtunnel, before being subjected to compaction.

In passing from the pressing station 14 to the compaction station 17 theroller speed is regulated so that the paper is subjected to a tensionsuch as to undergo maximum longitudinal elongation compatible with itsultimate strength, in order to obtain transverse contraction of thepaper, with a corresponding reserve of transverse extensibility.

Between the preforming station 15 and the drying station 16 there can beinterposed a drying and/or fusion station, preferably of infrared type.

In the station 17 the compaction, which occurs both in the longitudinaland in the transverse direction, is effected by passing the paper webbetween a pair of rollers (FIG. 6), of which the lower roller 36 is ofrubber and is driven at a certain speed, whereas the upper roller 37 isof metal and comprising a plurality of surface ribs 38, for examplecircumferential, and rotates at greater speed. Because of the pressureeffect and the configuration of the metal roller 37, cooperating withthe rubberized surface of the other roller 36, the paper web issubjected to undulation in the transverse direction and at the sametime, because of the different roller speeds, to a braking action by therubberized roller and to a consequent compaction in the longitudinaldirection.

On termination of this compaction stage the paper is subjected tofurther drying in the station 18 to achieve a dry content of about 85%,preferably 90%.

It should be noted that in passing from the compaction station to theexit of the drying station the roller speed is regulated (substantiallyconstant) such that no traction stress is applied, so that thelongitudinally compacted paper loses none of its longitudinalextensibility.

At the exit of the drying station 18 the paper web is subjected toglazing in the station 39.

The paper web obtained in this manner, in particular because of thebeating, impregnation, preforming and compaction treatment, presents ahigh degree of mechanical strength and of extensibility both in thelongitudinal direction and in the transverse direction, of the order ofat least 16% transversely and at least 20% longitudinally, for paperhaving weight for grams included between 100 and 150 g/m².

I claim:
 1. A method for producing extensible paper, comprising thesteps of:feeding a mix of vegetable fibers to a kneader member, mixingsaid mix with water in said kneader, beating said mix containing waterby rubbing the vegetable fibers in a multistep unit to obtain a pulphaving a degree of beating of at least 30° SR, transferring said beatenpulp into a flow chest, feeding said beaten pulp from said flow chestonto a paper web formation cloth with reduction of the water percentageby gravity and vacuum and forming a paper web, pressing said paper webwith further reduction of the water content of said paper web, initialdrying of said paper web to a constant moisture content of between 15%and 65%, compacting said paper web between at least a pair of rollers,one of said rollers being a hard material and having circumferentialsurface ribs and driven at first speed, and the other is of softmaterial with a smooth surface and driven at second speed, said secondspeed being less than said first speed, final drying of said paper webto a moisture content of between 4% and 15%, and glazing said paper web.2. A method as claimed in claim 1, wherein said final drying is to amoisture content of 8-10%.
 3. A method as claimed in claim 1, whereinsaid glazing step is done with a linear load of between 10 and 100kg/cm.
 4. A method as claimed in claim 1, wherein said glazing step isdone with a linear load of between 50 and 52 kg/cm.
 5. A method asclaimed in claim 1, further comprising carrying out said initial dryingstep until a moisture content of 45-50% is reached.
 6. A method asclaimed in claim 1, further comprising the step of impregnating, saidweb with additives in solution to increase the extensibilitycharacteristics of said paper web before initial drying.
 7. A method asclaimed in claim 1, further comprising a high density beating after saidbeating step.
 8. A method as claimed in claim 1, and said initial dryingstep said impregnated web undergoes a preforming step to deform said webin accordance with a defined profile which enhances the extensibility ofsaid web wherein between said impregnating step.
 9. A method as claimedin claim 1, further comprising the step of subjecting said paper web totension such as to give maximum longitudinal elongation compatible withits ultimate strength between said pressing step and said compactionstep.
 10. A method as claimed in claim 1, wherein no longitudinaltraction is applied to the paper web, after said compaction step, atleast until the moisture content of said web has fallen to below 4%. 11.A method as claimed in claim 1, wherein said kneader member is a highdensity kneader.
 12. A method as claimed in claim 1, comprising astorage step of said fibers, between the beating step, and transferringsaid fibers to the flow chest.
 13. A method as claimed in claim 1,impregnating said pressed web before initial drying step with additivesin solution to improve the subsequent preforming treatment.
 14. A methodas claimed in claim 1, wherein the vegetable fibers comprise long-fibrecellulose.
 15. A method as claimed in claim 1, wherein the vegetablefibers comprise short-fibre cellulose.
 16. A plant for producingextensible paper comprising:a kneader member where vegetable fibers arefed and mixed with water, a beating station for beating by rubbing thevegetable fibers in a multistep unit of discs of lavic material, a flowchest wherein the beaten fibers are transferred, a paper web formationstation with a cloth which receives said beaten pulp and reduces thewater percentage of said beaten pulp to form a paper web, a pressingstation receiving said paper web for the subsequent further reduction ofthe water content, a first drying station receiving said paper web fromthe pressing station for drying said paper web, a compacting stationcomprising at least a pair of rollers for compacting said dried paperweb, one of said rollers is a rubber material, the other roller which isof hard material, at least one of the hard rollers being provided with aplurality of circumferential surface ribs, a second drying station forfurther drying said dried paper web, and a glazing station for glazingsaid dried paper web.
 17. A plant as claimed in claim 16, furthercomprising an impregnation station between said pressing station andsaid first drying station.
 18. A plant as claimed in claim 17,comprising a preforming station downstream of said impregnation station.19. A plant as claimed in claim 18, wherein said preforming stationcomprises at least one preforming unit.
 20. A plant as claimed in claim19, wherein said preforming unit comprises an upper roller of shapedprofile and a smooth lower roller with smooth felt.
 21. A plant asclaimed in claim 19, wherein said preforming unit comprises a smoothupper roller and a smooth lower roller, with an interposed felt havingan external marking structure.
 22. A plant as claimed in claim 19,wherein said preforming unit comprises an upper roller of shaped profileand smooth lower roller.
 23. A plant as claimed in claim 19, whereinsaid preforming unit comprises a smooth upper roller, a smooth lowerroller, a first felt and a second felt of external marking structureinterposed between the felt and the lower roller.
 24. A plant as claimedin claim 17, wherein said impregnation station comprises of at least onetank containing an impregnating solution.
 25. A plant as claimed inclaim 24, wherein said impregnation station comprises elements forspraying said impregnating solution onto the paper web.
 26. A plant asclaimed in claim 16, wherein said kneader member is a high densitykneader and comprises a cylindrical tank having an invertedfrusto-conical base and housing a conical impeller comprising a relativehelix on its surface.
 27. A plant as claimed in claim 16, whereinbeating units alternating with storage chests form said beating station.28. A plant as claimed in claim 16, further comprising a second highdensity beating station of perforated ring type downstream of saidbeating station.
 29. A plant as claimed in claim 16, wherein said paperweb formation station comprises a cloth taut between return rollers andarranged to subject the pulp of water and fibrous raw materials toprogressive water extraction by gravity, vacuum or both.
 30. A plant asclaimed in claim 16, wherein said pressing station comprises pressingrollers and felts.
 31. A plant as claimed in claim 16, wherein saidfirst drying station comprises an infrared heater.
 32. A plant asclaimed in claim 16, wherein said first drying station comprises aplurality of hot roller.
 33. A plant as claimed in claim 16, whereinsaid first drying station comprises a hot air tunnel.